The research will deal with structure and function of leukocytes in various types of reactions. Studies will continue on intracellular mechanisms that permit certain parasites to survive and multiply within macrophages, and on modifications in these mechanisms in cells from immunized animals. Another line of investigation will be aimed at identifying the function of the novel, adherent, non-macrophage cells, called dendritic cells, in mouse lymphoid organs. The cell structures that are responsible for cell spreading on surfaces or retraction from the spread state will be studied with local anesthetics and chelators as reversible inducers of retraction, using time-lapse cinemicrophotography and electromicroscopy. Experiments are also planned to try to understand better the mechanism by which certain parasites or certain plant lectins alter the membrane of the macrophage phagocytic vacuole so as to render it resistant to fusion with lysosomes. Bibliographic references: Steinman, Ralph M.; Adams, Judy C. and Cohn, Zanvil A. Identification of a Novel Cell Type in Peripheral Lymphoid Organs of Mice. IV. Identification and Distribution in Mouse Spleen. J. Exp. Med. 141:804-820 (1975). Jones, Thomas C.; Len, Lucille and Hirsch, James G. Assessment In Vitro of Immunity Against Toxoplasma Gondii. J. Exp. Med. 141:466-482 (1975).